1. Field of the Invention
The invention relates to microwave heating and a utensil for producing browned and seared surfaces.
2. Description of the Prior Art
Microwave heating has become widely accepted for the preparation of a number of products. The microwave frequency energy is radiated within an enclosure from an energy source, such as a magnetron. The waves are radiated and reflected within the enclosure in free space and are distributed by such means as mode stirrers and the like. The microwave energy sets up a high frequency oscillatory movement of the molecules in a load to cause heating by molecular friction. The term "microwaves" is defined as electromagnetic energy radiation having wavelengths in the order of one meter to one millimeter and frequencies in excess of 300 MHz. Materials heated by microwave radiation have differing dielectric constant and loss tangent characteristics which result in heating in a varying pattern. As a result the varying absorption of the radiated energy causes the depth of penetration and surface coloration of the load to vary. It is difficult, therefore, to achieve a browned or seared surface similar to that produced by broiling without long periods of exposure which results in overcooking of the interior regions. Prior art techniques for browning include the incorporation of electric or gas broiling elements in the microwave oven. Another method involves the coating of the outer surface with a food additive having a high energy absorption characteristic. Another example of prior art teachings involves the use of lossy ovenware or utensils having selective heating capability by means of the use of conductive materials. Such conductive materials become heated from the absorbed energy and transfer this heat thermally to the supported load.
Still another example of prior art teaching is the utensil described in U.S. Pat. No. 3,857,009 issued Dec. 24, 1974 to G. MacMaster et al and assigned to the assignee of the present invention. This structure incorporates a load supporting means for converting and transforming the free-space energy waves into a finging electric field pattern having 180.degree. phase differential in close proximity to the exterior surfaces of the load. The means for converting and transforming the planar energy comprise alternating regions of high and low dielectric constant material. Providing high dielectric constant materials, such as those having a value of K38 or K50, tends to be costly. In addition in the patented structure an overall height of approximately one-half wavelength is required to arrive at the 180.degree. phase differential between the ends of adjacent elements. Further, in the patented structure the waves are directed through the dielectric material in varying degress to evolve the 180.degree. phase differential pattern. A continuing need exists for a microwave browning utensil which is relatively thin, lightweight and easy to clean.